Third, printing conditions and environment
In the printing process, the color of the ink film mainly depends on the concentration of the working ink and the transfer rate. For a specific substrate and a specific working ink, the concentration of the working ink is relatively fixed, and the transfer rate depends on factors such as the viscosity, the drying properties of the working ink, and the condition of the screen mesh, and also the printing speed and the printing pressure. , blade pressure, blade position, blade angle and environmental conditions (temperature and humidity) and other factors. Here we need to point out that the degree of influence on highlights, shadows, and midtones of prints varies when these conditions change.
1. The ink's working viscosity and drying speed change
The change in the viscosity of the ink can directly affect the color of the print. When the viscosity is high, the color concentration is relatively high, whereas when the viscosity is low, the color concentration is also relatively low. This effect is sometimes very obvious and must be given high priority. Due to the requirements of the printing process, the working viscosity of the ink should be moderate. When the working viscosity of the ink is too large, it is not conducive to the transfer of the ink, and it is easy to produce a paste-type phenomenon. When the working viscosity of the ink is too small, the color of the printed product is prone to static electricity, which is also unfavorable to the control of the ink color.
The change in viscosity has a subtle effect on the flow area created by the flow of the ink or the flow of the dots, especially in multi-color printing. Therefore, when the printing speed is constant, it is required that the viscosity of the ink be consistent from beginning to end, and it is better to use an automatic viscosity controller.
When the proportion of the solvent in the ink changes, the drying speed of the working ink changes, which in turn affects the ink transfer rate and the printing color. For some inks with a small amount of ink, after being used in the ink tank for a period of time, due to the natural volatilization of the solvent, not only will the working viscosity of the ink be affected, but also the ratio of the solvent and the volatilization speed will be affected, so the solvent must be added in time.
2. Temperature and humidity changes (environment)
At room temperature, the liquid temperature of the ink changes greatly, the flow state changes, and the hue changes. This is because the change in temperature affects the viscosity of the ink. At the same time, the dry weather with high temperature and low humidity will have a significant effect on the ink transfer rate of the highlights. Therefore, when printing high-end products, the temperature and humidity of the printing shop must be controlled anyway. In addition, when using the ink in winter, warm up in advance to reduce the temperature change of the ink itself.
3. Synthetic color separation
This phenomenon is likely to occur in a mixture of light colors and various colors such as organic colors and inorganic pigments. In particular, when the ink flow in the ink tray is too slow, sedimentation further promotes the separation of the synthesized colors, so care must be taken to make the ink circulate smoothly. In addition, if the mixing is not sufficient during the color mixing, the concentration of a certain color tends to increase as the printing progresses. Therefore, it is necessary to perform thorough stirring using a machine such as a dissolver or other method before the printing.
4. The ink deteriorates and deteriorates the color
Due to long-time printing, the solvent composition of the ink is changed, or the ink in the air is mixed into the ink to cause deterioration of the ink and the color becomes deteriorated.
In order to prevent temperature drop due to heat of vaporization, a dry solvent may be used in combination. In the case of high temperature and high humidity, there may be a certain amount of water mixed in the ink, so in the event of an abnormality, new ink should be added or replaced completely. The remaining ink that has been used many times must be filtered or discarded because it contains a lot of dust.
5. Other printing conditions
The discoloration of the printing may occur due to coincidence of the dots and the irregular wind blowing on the layout, or changes in printing speed and drying speed.
IV. Postpress Processing and Environmental Impact
1. Post-printing process changes
The use of different post-printing processes (such as extrusion compounding and dry compounding) also has a certain effect on the hue of the prints. Simultaneous extrusion of high temperature composites can sometimes cause pigment changes. In addition, when the hot seaming is performed at a temperature of 200° C. or higher, the hue of the heating section may become deep. This is due to the increase in coloring power due to the formation of microcrystals during recrystallization due to melting and cooling of the heating colorant.
2. Post-printing substrate changes
Changes in the post-printing substrate, such as changes in the aluminum layer of the aluminum film, directly affect the overall hue. Of course, the color deviation of the printed substrate will also have a slight effect on the overall hue.
3. Photochemical reaction
When the light resistance and weatherability of the colorant are poor, the color may change due to the photochemical reaction.
4. Content impact
After the contents are packaged, when the contents contain volatile and migratory active materials, the color of the packaged print may also change due to corrosion. Therefore, the appropriate ink should be selected according to the end use of the package (or negotiate with the ink manufacturer).
Fifth, the naked eye and color control instruments
The customer wanted a high degree of consistency in the ink of the packaging print, but the result of each person's observation of the color was not exactly the same. What customers need is persuasive data, which is quantitative data for colors. They do not want to say that it is blue but want to say what the L, A and B values ​​are. Although the naked eye is good at color, it is not good at remembering colors (if you don't put the products together and you can't accurately measure the color with the naked eye), so the naked eye is no longer considered as a reliable guarantee for determining the color of ink. The instrument shows its uniqueness. importance. From the naked eye to the densitometer, colorimeter and then to the spectrophotometer. Later, it developed into the current bench-top spectrophotometer. Nowadays, various technologies are sufficient to compensate for the eye fatigue effect, persistence of vision, visual memory and other adverse effects on color observation, and the color measurement result is more accurate. Of course, it takes time to use color control instruments in large areas in China.
As part of the color management process, color detection is an indispensable step and the first step in color management. At the same time, since the stability of the color during the operation of the printing press will be affected at any time, the use of an on-line color control device to monitor the color stability will be an important development trend.
other
1. For plastic intaglio printing, there is no continuous and complete color quantitative control system from plate making to proofing. Therefore, it is necessary to simulate the printing conditions and standardize the proofing as much as possible during the proofing process.
2. Printing proofing must be performed under normal printing conditions, otherwise it will cause difficulties in the pursuit of samples. For example, when the squeegee is too close to the imprinting line during the proofing, it is difficult to handle or adjust when the squeegee related failure occurs in the official printing. If the sample is printed with the squeegee away from the imprinting line, it may be difficult to transfer the ink in some shallow places during the formal printing. The printing spots will appear on the printed product. When removing these flowers, the ink will certainly change.
3. The control of printing ink is mainly to control the mixing effect of multiple colors. In the specific operation, it is necessary to accurately determine which color has changed. Be careful not to patronize a local hue, but forget to care about their interrelated color changes.
4. Strengthen the color management of printing
Sometimes the colors look the same even under normal lighting, but they don't look the same under another light source, so you should use a standard light source for color or color comparison.
The content of color management is very rich. It is very important for the plastic packaging and printing industry to develop an effective color management system.
5. Sometimes, the printing process may not always be ideal. Some factors must be adjusted and a new balance must be reached after this adjustment: Hue will change in the following situations. Therefore, the color sample must be checked before starting the job.
(1) Change in viscosity (2) Renew blade (3) Change speed (4) When supplementary ink is added (5) When solvent is added
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